Apparatus for treating impure liquids



Feb' 1, 1944 D. c. REYBOLD ET AL 2,340,848

APPARATUS FOR TREATING IMPURE LIQUIDS Filed Deo, 18, 1941 mmm atented Fehr. i@ l@ eel PiPES FR TREIENG MURE in ist Douglas C. Reybold, Northbrook, mi., and lintliony ll. Fischer. Mascot, N. Y., aselgnors to 'Ellie liliorr Gompany, New York, N. Y., a corporation or Delaware l2 line.

This invention relates to the purication of polluted or impure liquids such as sewage, trade wastes and the like. In such purication, one recognized method in use today involves the use of a bed ofi discrete material containing biologicai organisms which include a wide range thereof from bacterial ora up to animal life as enem. plied by worms. The biological organisms are depended upon to convert the unstable and putrescible or septicai organics that render the liquid impure linto stable innocuous substances. The organisms are also depended upon, at least in part, to render nonsettleable suspended solids more readily settleable.

The invention has relation to what is known in the sanitary engineering art, as trickling iilter` beds. Of late it has come to be accepted that it is desirable to supply liquid to be treated in iilterbeds at rates that would have been considered a few years ago as' dangerously high. 'This invention has especial application to high-rate iilter-beds. An outgrowth ci the high-rate filter-bed is that there be associated with it a recirculation systeni whereby liquid being treated can be repeatedly or continually recirculated through the lterbed. This permits the continuous dosing on the filter-bed of a liquid more uniform in character, and reduces the extreme uctuations in flowc ln most systems one or more detention tanks are associated with the lter and the recirculation system may be so arranged that lter efiiuent is recycled back to the detention tank or primary clariiier preceding the filter; or detention tank or secondary clarifier eiiiuent is recycled back to the lter which precedes the detention tank. In either case the iilter is dosed at such a rate with liquid containing micro-organisms favorable .to the promotion of optimum biological conditions in the lter bed that clogging of the lter bed is avoided due to a continuous sloughing oii of the solids that would normally tend to accumulate. At the same time, the recirculation from the iiiter to the primary clarifier or from the sec ondary clarier to the filter dilutes the strength oi the liquid fed to the filter so that it becomes more amenable to treatment. The detention tank is usually provided with means for mechanically cleaning it oi deposited sediment to guard against septicity oi the sediment, and the tank is one from which overilows a ciariiied efiiuent.

In recirculation types of trickling lter plants involving a illter and a clarifier in closed-circuit, the rate of filter application may be multiples of the rate of new feed incoming to the system. The new iced may be introduced at any suitable dit point. Likewise, the eiduent may be discharged from the system at any desired point.

Among other advantages realizable, mention is made or the fact that recircuiating tric lter systems gave the unexpected result oi permii-.ting the use of nlter beds only three ieet or less in depth which could be operated at high om ciency. This permits the use of high :dlter load ings iigured as pounds of E. O. D. per cubic yard of ltering media per day. lt has been found that the eiciency of recirculating type filters increases as the amount ci' recirculation increases.

' The high rates# of dosing tend to prevent clogging of the interstices between the discrete stone par ticles of the illter media; ample food is brought to the immotile biologic organisms oi the bed; their excretions are washed away therefrom; and some of the organisms are washed into the clarin ner so that biologic oxidizing activity ci the septical organics is continued in the clarier in addition to the bed Where it is initiated. This meth od oi operation also ireepg down. the :filter-ily nuisance and further keeps down the odor nui sance.

The disadvantage ci' the use of such high re circulation rates, however, is that a large and expensive piping system is required to convey the claried eiiuent to the triekling filter bed, other Wise a high loss oi head will result, possibly re quilting the pumping to the beds.

According to the present invention a liquid treating system is employed wherein a relatively shallow bed of discrete material is disposed in encircling arrangement with respect to a liquid-= receiving tank of a type which may :function as a ciariiier. The tank is furthermore of a type having supernatant outnovv ineens disposed about and leading irorn the upper marginal ree gions thereof. Also according to the present in@ vention such shallow bed of discrete material functions as a tricirling lter-bed extending to an elevation whereby gravitating filter emuent can be collected and, according vto operative requirements, pumped bacls into the tenis suba stantially against only a minimum or relatively low back pressure head. The top o.' the iddtera bed is disposed at elevation sufficiently loer so that there can always be realmed a gravity of clarified liquid iront. the supernatant outdoor means, to Wit, :from the region about the upper marginal portion of the tenir, and consequent spraying of the gravityaconveyed liquid over the trickling iilter-bed in a relatively uniform. dism tribution. The use oi the shallow bed encircling the tenis with its surface lower than the liquid level in the clarifier tank assures the gravity feed to the filter-bed direct from the clarifierL through a conducting and distributing means which may be relatively simple and of low cost. This may be through a means embodying a number of relatively uniformly but horizontally spaced indi vidual pipes extending downwardly from the upper marginal regions of the tank and of which the pipes terminate in spray heads or other suitable liquid diffusing delivering means. Each of these pipes is preferably provided with a valve` therein whereby the flow therethrough and therefrom may be adjusted to operative requirements or according to the particular type of operation decided upon. The use of a shallow bed encircling the clarifier assures gravity feed to the filter-bed direct from the clarifier through a pipe system of low cost.

In the practice of straight or once-through filtration, over which the recirculating type was an improvement, the overfiow from aprimary clarifier passes through the trickling filter where biological sludge is formed and` the filter-treated liquid enters a secondary clarifier where separation of the liquid is effected into settled sludge and effluent.

The straight fiter must be dimensioned and operated to effect as completely as possible the bilological purification and coagulation of sewage impurities within the interval allowed -for a single passage through the filter. This necessitated a large volume of filter media and a depth of the filter which was accepted to be from 6 to l2 feet. This depth was so much the standard that it became a custom with sanitary engineers in specifying filter loads or capacities, to disregard the factor of filter depth as a constant and refer to filter capacity in terms of area or horizontal expanse only. Then there was accepted for filter capacity or load the dimension m. g. a. d.; that is, million gallons per acre per day."

Since the patentees are working e:sentially with a shallow filter-bed in circuit with a detention tank or clarifier, the invention has for one of its objects an arrangement for the .proper integration of these elements while they are concentrically disposed, with the clarifier or detention tank in the center and the filter-bed or beds encircling the clarifier. Another object is to modify this arrangementl for two-stage treatment. And a further object is to provide means for returning the filter-bed discharge back to be mixed with the liquid in the clarifier tank. A still further object is to provide means in this assembly for thoroughly distributing over the filter-bed area, the liquid passing thereto. Another object is to make such an assembly into an esthetically attractive design, for sewagetreating plants are being located increasingly in parks or other public spaces of municipalities that are more or less landscaped.

Another object is to provide an arrangement whereby the liquid from the clarifier can be supplied to the filter-bed and distributed thereover substantially uniformly while under a conu stant head. And a still further object is to devise apparatus for dosing the filter-bed direct from a launder of the clarifier.

As illustrative of a manner in which the invention hereof may be realized. reference is made to the accompanying drawing constituting a part of this specification.

In said drawing:

Figs. l and 2 are, respectively, plan and verti- Weir.

cal sectional views diagrammatically illustrating two filter stages provided in a trickling filter bed disposed around a primary clarifier and ar ranged in operative association with a subsequent secondary clarifier tank. 1

The application upon which this patent is based originally contained Figs 1 to 7, inclusive. The descriptive matter particularly pertaining to original Figs. 1 to 5, inclusive, has been made the basis of divisional application, Serial No. 499,812, filed August 24, 1943, and hence those figures and certain description relating thereto have been eliminated herefrom.

Original Figs. 6 and 7 nave been re-numbered and now stand as Figs. l and 2 hereof. The substance of the descriptive matter thereof continues herein. For a fuller description and amplification of detail features of concentrically arranged sedimentation units and trickling :lter beds made of discrete material, one may refer to said divisional application and the patent thereupon, to wit, Patent No. 2,340,842, granted Feb. l, 1944.

While the drawing hereof is diagrammatic, nevertheless it is believed that the disclosures thereof and the descriptive matter pertaining thereto avoid any necessity of a more extended amplification and description, particularly in view of the relatively detailed disclosure of the functioning parts available in the aforementioned divisional application and the patent granted thereupon.

In the figures hereof there is shown at A, sometimes herein referred to as stage A, the combination of a primary clarifier tank and a trickling filter bed of discrete material arranged in concentric relationship with respect to each other. The filter bed is designated as 33 and embodies a primary treating section 33 and a secondary treating section 33h as will hereinafter more fully appear.

In the construction shown the clarifier tank H is cylindrical and the trickling filter bed is annular in plan and surrounds the tank. At B there is a clarifier tank herein referred to as a secondary clarifier functioning as a sedimentation unit of a secondary treatment stage that is brought into operative relationship with respect to the primary clarifier and a secondary treating section of the trickling filter bed just mentioned. The primary clarifier or sedimentation tank, as it may be called, is designated Il and the secondary clarifier or sedimentation tank is desfignated as HS. Each of these clarifiers as shown is provided by a tank embodying a marginal or cylindrical side wall l2, and a tank bottom I3, the latter of which provides a sedimentqeceiving sump 23 from which sediment is conveyed at the will of an operator through a valve-controlled discharge pipe or conduit 2li. Each of these clarifiers has a centrally located feed-receiving means in the form of a cylindrical baflie wherein incoming liquid to be treated in the particular clarifier is initially received and from which it is distributed into the body of liquid within the clarifier. This feed-receiving and distributing meanssometimes referred to as feedwell-for the primary clarifier, is designated as il and for the second clarifier as HS.

The upper edge of the marginal wall of each tank is horizontal and functions as an overflow supernatant liquid from any one of the tanks must ovefiow this edge or Weir provided thereby before being received into a peripheral launder corresponding thereto provided at and about the upper edge portion of the tank. Such peripheral launder is sometimes referred to as an overflow-receiving launder and the peripheral launder for the primary clarifier tank is designated as 2s, while that for the secondary clarifier tank is designated as 26. The overiiow edge or weir provided for the primary tank is designated as 25, while that for the secondary tank is designated as 25S. The overfiowed liquid received within the primary launder has gravity iiow therefrom through descending horizontally spaced valve-controlled pipes 30 terminating into distributing heads 32. The valves for controlling the pipes Si] are designated as 3l. These descending pipes have relatively uniform horizontal disposition with respect to each other whereby for. the trickling filter bed sections served thereby there is realizable a relatively uniform distribution of liquid from the launder to and over each particular filter-bed section.

The trickling filter bed sections function as biologic treating means. The trickling filter bed is composed of any suitable material usually employed in beds of this type and it will be noted that the trickling lter bed which is annular in plan is divided by vertically-extending partition members i@ and 113 into that which may be referred to as a primary filter bed, or lter--bed section 33 and a secondary iilter bed or filterbed section 33". It will also be observed that low partitions or transverse weirs are provided in the primary clarifier launder 2t at T3 and i3 whereby that launder is divided into launder sections and 28h respectively, corresponding to the primary filter-bed sections 33 and 33h. For this reason the launder section 28e is sometimes reierred to as a primary launder section and b is sometimes referred to as a secondary launder section. The launder section 28h may also be viewed as a commingling launder section, because there is a commingling therein of liquids received from different sources.

incoming liquid or sewage to be treated is supplied from a main pipe line or conduit i3 and is delivered into the primary clarifier, to wit, within the feed-receiving means or cylindrical distributing baiiie ii thereof. As will hereinafter appear there may be introduced into this feed line 33, thus into the primary clarifier, recirculated eiiluent derived from the primary trickling-fllter seciton 33e. All of the liquid thus supplied for treatment into the primary clarifier, including that returned to the primary clarifier for re-treatment therein, must ultimately pass from the clarier as supernatant liquid over the Weir 25 into the launder 28. Of the supernatant that overiiows into the launder 2B, some will pass into section 28e and some into section Ztl thereof.

The partial partitions i3 and i3 may be described as overiiow barrier means extending upwardly from the bottom of the launder 2li and terminating at elevation lower than that of the overflow weir edge 25. With such an arrangement there can take place a flow from one launder section past said barriers or transverse weirs 83 and i3' into the other launder section if and when liquid in one of said launder sections exceeds and rises higher than that in the other section and to elevation higher than the top of said members i3 and 'i3'.

The euent passing through the trickling filterbed sections 33B and 33h is received on a sloping iioor 'dii underlying and common to said filter-bed sections. In the floor there is a sump for and corresponding to each of the iilter-bed sections,

namely, a sump d@ for receiving filter eiiluent from the primary filter-bed section 33 and a sump do for receiving eiiiuent from the secondary filter-bed section 33h.

It has already been pointed out that since the weir 25 of the primary clarifier has the same ele vation for its entire periphery, some of the liquid overilow of this Weir will pass `into the primary launder section 23 and some into the secondary or commingling launder section 2th.

Since the tops of the transverse barriers or partitions i3 and iti do not extend upwardly to an elevation as high as that of the overow weir 2S, it will be manifest that by restricting the outiiow from the primary launder section Q through the valve-controlled pipes il@ therefor, to wit, by partially closing the valve 3l of said pipes, it is possible to provide for a delivery of only a limited quantity of liquid from the primary launder section 28e to the corresponding primary trickling lter-bed section 33 and there follows a consequent flow of excess liquid from the primary launder section 28a over and past the trans-verse barriers or weirs i3 and i3 into the secondary or oommingling launder section 2th from which the liquid flows through the descending valvecontrolled pipes il@ corresponding thereto for distribution to and over the secondary filter-bed sections lb-this assuming that the valves 3i of the last mentioned set of` pipes 3d are open. In this manner one can regulate or control the now distribution to the different trickling iilter-bed sections.

Mention has heretofore been made that the eiiiuent from the primary trickling iilter bed 2531 can be passed back to the feed-distributing means or cylindrical baiile l? of the primary clarifier. This is accomplished by a pump drawing filter eiiiuent from the primary collecting sump t@ and delivering the pumped efiiuent into feed line (i3 at the discharge side of a check valve 55. This check valve 55 has been provided in said ieed line i3 to assure a forward ow of pumped eiiiuent into the feed-receiving and distributing means H, or as otherwise expressed, to avoid a iiovv of pumped eiiluent in a backward direction counter to the flow of incoming sewage.

Respectng the secondary clarifier MS and its functional connection and arrangement with re spect to stage A, it will be observed that the secondary iilter-bed elluent collects in a receiving pocket or sump dii' and is passed or dis charged therefrom through feed-line 63S into the feed-receiving and distributing means HS of the secondary clarifier` Incident to this transfer or discharge'of secondary eiiiuent from sump dii there is a consequent overiiow of supernatant liquid over and past the marginal top edge or Weir 25S of the secondary clarifier into the overow receiving launder thereof from the latter of which there is an outflow into a receiving means providing a well or pocket it.. ln this pocket or well it a certain quantity of liquid is retained whereby it is available for recirculation through the medium of a pumping means embodying a pump il, a suction line i@ leading thereto fromA said well Ii8 and a pump discharge line i2' by which the liquid thus pumped is delivered into the aforementioned launder section 281 of the primary clariiier and wherein the returned pumped liquid is commingled with the liquid already therein. In this way the secondary clari-s fier HS is brought into cyclic arrangement with the secondary trickling iilter-bed section 33h.

It will be noted that while tank l IS functions as a clarification tank and it is also practicable to make it large enough to function as a storage and detention tank for holding a substantially large quantity of liquid undergoing purification as well as clarication. In such instance it detains as stored liquid, liquid which is progressively treated against septicity and which treatment may be continued to further purify the stored liquid. The return of some of the liquid from-this secondary stage tank back to the trickling filter of the primary stage enables such utilization of the secondary tank and the accomplishment of such type or degree of sewage treatment as may be desired in or for the system as a whole.

From the upper portion of the pocket or well 18 there extends an outflow pipe 'l5 by which there is a release from the system hereof of clarified liquid which would otherwise accumulate therein.

Concerning the system just described and from the foregoing it will be noted:

That with respect to stage A thereof tank il and section 33n of the trickling filter-bed are in cyclic arrangement and may be viewed as functioning as a primary cyclic unit of a multi-stage treating system;

'Ihat the incoming liquid supplied for treatment is initially delivered into tank l I of the primary cyclic unit;

That trickling filter-bed section 33b of stage A has distributed thereover liquid passing thereto from said primary cyclic unit;

That trickling filter section 33h of stage A and tank I i of stage B are in cyclic arrangement whereby they function as a secondary or subsequent cyclic unit;

That the cyclic treatment of this secondary cyclic unit can be carried out to attain the degree of clarification or purication desired; and

That the treated effluent is released from the system as outiow from the tank li of stage B its ultimate release being along pathway l leading from the sump or well 'i3 that receives liquid overilowing Weir 25 through the medium of launder 2S.

In connection with each of the combined settling tanks and trickling filter-bed units shown in the drawing hereof, it will be noted that the depth of each trickling filter-bed is relatively shallow as compared with the depth of the tank. As shown the construction providing the bottom or i'ioor of the trickling filter-bed is shown practically as an extension of the bottom of the tank. With this arrangement a feed-supply pipe or influent conduit such as 53 can, without undue errcavation, be located so as to extend directly below the .door of the filter as well as that of the settling tank.

The elevation of the top surface of each trickiing filter-bed is substantially lower than that of the surface level of the liquid in the tank with which lt is associated, or, as otherwise expressed, lower than the launders from which liquid is gravitationally passed through valve-controlled doumiiow pipes such as 3G and Eli for distribution over the top of the bed.

In the instance of the form illustrated the height available for effecting this gravitational transfer is substantially that difference in elevation between the upper surface of the trickling filter-bed on the one hand and the elevation or Yapproximate elevation of the overiiow Weir 25.

This difference in elevation or static head availu downow pipes for conducting rom launder some of the supernatant outilowed Linn-ein and for distributing the liquid thereby conducted upon the primary filter section, means comprising horizontally-spaced downiicw pipes for conducting .from the launder some of the supernatant able is preferably of the order of from 3 to 4 ft. of water.

We claim:

l. Apparatus for treating impure liquids comprising in operative combination and arrangement a primary settling tank, a secondary settling tank, means for passing sedimented material from the primary tank, means for passing sedimented material from the secondary tank, means for feeding impure liquid to the primary tank, a launder disposed for receiving supernatant liquid outowing from the primary tank, a tricklng lter-bed of discrete material disposed about the primary tank, partitioning means functionally dividing the filter-bed into a primary lter section and a secondary filter section, means providing a oor structure for said bed and having a sump for each lter section and serving for receiving eiiiuent from the lter section to which it corresponds, valve-controlled means for passing from the launder some of the supernatant outiiowed liquid therein and for distributing the liquid thereby passed upon the primary filter section, means for passing from the iaunder some of the supernatant outowed liquid and for distributing the liquid thereby passed upon the secondary filter section, means for transferring eiiiuent from the sump corresponding to the primary filter section into the primary tank, means for transferring filter eiiluent from the sump corresponding to the secondary filter section to the secondary tank, means providing a well associated with the secondary tank and serving for receiving as outflowed liquid supernatant liquid from the secondary tank, means for releasing as emuent from the well some of the liquid therein and means for passing from said well back to the secondary filter section some of the liquid within said Well.

2. Apparatus for treating impure liquids as defined in and by claim l, and according to which the primary tank is circular in plan, the launder extends along the upper marginal edge or" the tank, and has overflow partitioning means rising from the bottom of the launder but termina-t ing at elevation lower than any edge oi the launder whereby some of the liquid in the launder can pass as overiiow from a section of the launder corresponding to the primary section of the illter-Led to a section of the launder corresponding to the secondary section of the filter-bed.

3. Apparatus for treating impure liquids comprising in operative combination and arrangement a settling tank, means for passing sedimented material from said tank, means for feeding impure liquid to the tank, a marginal launder disposed for receiving supernatant liquid outiiowing from the tank, trickling filter-bed of discrete material disposed about the tank having substantially les depth than that of the tank and whose top surface is at elevation substantially lower than that of said launder, partitioning means functionally dividing the lter-bed into a primary filter section and a secondary iter section, means providing a floor structure for said bm having a sump for each filter-bed section and serving for receiving iilter Viliuent from the nl bed section to which it corresponds, prising horizontally spaced valve -c outflowed liquid and for distributing the liquid thereby conducted upon the secondary lter section, means for transferring eiliuent from tli'e sump corresponding to the primary filter section to and into the tank, and means for transferring filter eiliuent from the sump corresponding to the secondary filter section.

4. Apparatus for treating impure liquids as defined in and by claim 3, and in which the launder is provided with transversev overflowl partitions l rising to elevation lower than that of the upper edges of the launderand disposed for dividing the launder into a section from which liquid is supplied directly to the primary filter-bed section, and a section that also receives liquid overowing 1 said partitions and from which liquid is supplied directly to the secondary filter-bed section.

5. Apparatus for treating impure liquids comprising in operative combination a settling tank,

means for passing sedimented material from said`\ tank, means for feeding impure`liquids to the apparatus for treatment therein, a launder` having partitioning means dividing it into functionally separated sections of which one ls a primary section while'another is a secondary section, which 25 trickling lter bed section of discrete material, a '30 secondary filter bed section of discrete material, means providing a floor structure having a sump for and corresponding to each lter bed section and serving for receiving filter eiiiuent from the filter bed section to which the sump corresponds,

primary lter bed section back to the tank, means 40 for passing liquid from the secondary laundersection and for distributing it upon the secondary lter section and means for discharging lter eiuent from the sump corresponding to the secondary lter section.

6. Apparatus for treating impure liquids which comprises a two stage plant of which the rst stage comprises a clarifier tank and a circumjacent trlckling filter-bed having a bottom that supports discrete iilter media, a sump section in said bottom, a pump for drawing filter discharge from the sump, means for removing sediment from the tank, a. conduit for the supply of feed liquid to the tank, a connection between the pump andthe conduit, launder means for determining the liquid level in the tank and for receiving liquid overiiowing thereinto from the tank, gravity operated iiow means for conducting liquid from the launder means to the filter-bed and for distributingsuch liquid thereover; the

second stage comprising a secondary clarifier 7. A unit for treating impure liquids comprising in operative combination a settling tank, means for passing sedimented material from said tank, means for feeding impure liquids to the unit for treatment therein, a launder having par- 7 arate contiguous sections of which'one is la primary section andthe other is asecondary sec,y tion, which said launder sections are disposed for y simultaneously receiving supernatant liquid overflowing directly thereinto from the tank past Weir edge portions of said launder sections, which said partitioning means extends upwardly to an elevation substantially higher than that of the lower interior portion of the launder but has an over-- flow top'portion at elevation lower than that of said overflow Weir edge portions of said launder sections, a kprimary trickling filter bed `section of f of the primary lter bed section back to the tank,v

means for passing liquid from the secondary launder section and for distributing it upon the secondary filter bed section. and means for discharging lter eilluent from the sump corresponding to the secondary iilterbed section.

8. A unitfor treating impure lquidas dened mand by claim 7,"according to which the tank is circular in plan, accordingv to which the launder extends along the marginal edge of the tank and has a member circular in plan providing an overflow launder Weir having the same elevation for its entire length whereby of the supernatant outowing from the tank some will pass one portion of said weir as overflow into the primary launder section and some will pass another portion of said Weir as overflow into the secondary launder section, and according to Wlilch the partitioning means comprises transverse members extending across the launder for the full width thereof and upwardly from the bottom of the launder and of which at least one of said transverse members terminates at elevation lower than that'of the overflow Weir and serves as a liquid-overflow means between the primary and secondary launder sections and over which some of the liquid when high in one of the launder sections can pass into the other launder section.

9. A unit for treating impure liquid as defined I in and by claim 7 in which the launder has a memberproviding an overflow Weir having the .same lelevation for a substantial length thereof whereby of the supernatant liquid outiiowing from the tank some will pass the weir as overflow into the primary launder section and some will pass the Weir as overflow into the secondary launder section; and in which the partitioning means extends upwardly from the bottom oi' the launder and across trie launder for the full width thereof and. terminating at elevation lower than that of the overflow Weir for thereby dividing tne launder into functionally distinct primary and secondary Y launder sections wmle providing an overflow means between said sections over which some liquid in one oi said launder sections can overflow into the other oi said launder sections.

10. -A unit for treating impure liquids comprising in operative combination and arrangement a settling tank, means for passing sedimented material from said tank,- means for feeding impure liquidv to the unit, means .providing a primary launder section disposed for receiving supernav tant liquid outriow from the tank, means provid-k titioning means dividing it into functionally seping a secondary launder section for receiving supernatant liquid outflow from said tank, said launder sections having members providing overflow weirs at substantially the same elevation and over which supernatant liquid from the tank simultaneously passes on its way to said launder sections, a primary trickling filter bed of discrete material, a secondary trickling filter bed of discrete material, means providing iloor structure for said beds and having a sump for and corresponding to each filter bed for receiving filter emuent from the filter bed to which the sump cor- Y responds, primarytransfer means for passing liquid from the primary launder section to the primary trickling filter bed, a secondary liquid transfer means for-passing liquid from the secondary launder section to the secondary trickling nlter bed, means for transferring into the tank filter eniuent from the sump corresponding to the primary filter section, and-means for discharging iliter eiliuent from the sump corresponding to the secondary lter section.

11. A unit according to claim 10 in which the launder sections have liquid transfer means between them for releasing excess liquid from one launder section into the other.

12. A unit for treating impure liquidsscomprising in operative combination and arrangement a settling tank, means for feeding into the unit impure liquid to be treated, means for passing sedimented material from said tank, a marginal launder disposed for receiving supernatant liquid outilowing from the tank past a member providing an overow weir edge, transversely exten/ding partitioning means within the launder rising from the bottom thereof for dividing the launder into functionally separate primary and secondary launder sections, said partitioning means terminating at an elevation lower than that of the aforesaid overiiow Weir edge whereby according to operating conditions there is permitted an overdow release of liquid from one launder section past said partitioning means into the other launder section, a primary trickling illter bed of discrete material,l a secondary trickling filter bed o! discrete material, means by which liquid from the primary launder section is passed to and distributed over the primary trlckling filter bed, means by which liquid from the secondary launder section is passed to and distributed over the secondary trickling filter bed, means for collecting primary filter eiliuent and for conducting at least some of the same back into the-tank, and means for collecting secondary filter eiliuent and for releasing a quantity of the same from the unit.

DOUGLAS C. REYBOLD. ANTHONY J. FISCHER. 

